Exhaust systems of internal combustion engines are generally fitted with an exhaust-gas aftertreatment device which may comprise in particular one or more catalytic converters, for example an oxidation catalytic converter. For optimum functioning, many exhaust-gas aftertreatment devices utilize, at least intermittently, an exhaust-gas temperature higher than that attained during normal operation of the internal combustion engine. The required temperature increase may be realized for example by an exothermic reaction of unburned fuel, which is contained in the exhaust-gas flow, on an oxidation catalytic converter element.
For the enrichment of the exhaust-gas flow with fuel, it is known for fuel to be injected directly into a cylinder of the internal combustion engine a post-injection which takes place after the actual combustion process. The problem may however arise here that some of the injected fuel passes into and thins the engine oil used for lubricating the internal combustion engine, such that the lubricating action of the engine oil is diminished.
Therefore, fuel is normally injected directly into an exhaust tract of the internal combustion engine for the enrichment of the exhaust-gas flow with fuel. For this purpose, an injection nozzle is provided with fuel that is at a relatively low pressure of typically 5 to 10 bar, injected directly into the exhaust-gas flow or sprayed onto a heating element which assists an evaporation of the fuel before it is introduced into the exhaust-gas flow. The injection system required for this purpose increases the complexity and the costs of the overall system.
In U.S. Pat. No. 4,815,423 and US 2011/0126799 A1, an outlet valve is opened during a part of the compression stroke of a cylinder in order to discharge some of the air contained in the combustion chamber. The outlet valve is subsequently closed, the remaining air is compressed, and fuel is introduced into the cylinder and ignited. In this way, an enrichment of the exhaust-gas flow with fuel is not realized.
It is an object of the present disclosure to propose a method and a system for controlling an internal combustion engine, in which method and system an exhaust-gas flow is enriched with fuel, wherein the disadvantages mentioned above should as far as possible be avoided.
In a method according to the disclosure for controlling an internal combustion engine which has at least one first cylinder, it is provided that, during a compression stroke of the at least one cylinder, an outlet valve of the first cylinder is open for the introduction of a fuel-air mixture from a combustion chamber of the cylinder into an exhaust tract of the internal combustion engine. The internal combustion engine is in particular an applied-ignition engine. Here, the fuel-air mixture may have been produced outside the combustion chamber of the cylinder, for example, an intake pipe injection, and introduced into the combustion chamber; the mixture may however also have been produced by a direct injection of fuel into the combustion chamber. If the internal combustion engine has an exhaust-gas recirculation system, the fuel-air mixture may contain at least a fraction of recirculated exhaust gas. Furthermore, the outlet valve may be open during a discharge stroke of the at least one cylinder.
By virtue of the fact that the outlet valve of the cylinder is, according to the disclosure, open at least for part of the duration of the compression stroke, the mixture situated in the combustion chamber of the cylinder can pass into the exhaust tract. In particular, the movement of the piston of the cylinder during the compression stroke, said movement taking place in a direction for a reduction in the volume of the combustion chamber, causes the mixture to be forced into the exhaust tract of the internal combustion engine, for example into an exhaust manifold, from where said mixture passes for example into an exhaust-gas aftertreatment device. An enrichment of the exhaust-gas flow with unburned fuel may be attained in this way, whereby for example improved operation of an exhaust-gas aftertreatment device and/or an increase in the exhaust-gas temperature at an oxidation catalytic converter element may be attained. Furthermore, opening of the exhaust valve during the compression stroke may allow for release of unburned fuel into the exhaust tract nearly immediately, minimizing possible accumulation on combustion chamber walls and may further allow the timing of release of the unburned into the exhaust tract to preempt exhaust blowdown by adjacent cylinders. This may provide a reservoir of unburned fuel to act as a reducing agent within an exhaust gas aftertreatment device prior to exhaust of NOx- and other emissions by adjacent cylinders.
In addition to the at least one first cylinder which is operated as described above, the internal combustion engine particularly preferably has at least one further, second cylinder which is operated in the normal mode, that is to say the outlet valve of the second cylinder is closed during the compression stroke. By virtue of the fact that the outlet valve of the first cylinder is substantially open during the compression stroke and the fuel-air mixture is conducted into the exhaust tract, it is the case that, at the end of the compression stroke, no mixture, or an insufficient amount of mixture for power generation in the expansion stroke, is available in the combustion chamber of the first cylinder. In the case of an applied-ignition engine in particular, it may also be provided that no ignition takes place. The first cylinder is thus not utilized for the power generation of the internal combustion engine, and is thus “deactivated”. A selective cylinder deactivation is realized by the opening of the outlet valve during the compression stroke of the first cylinder. By virtue of the fact that the first cylinder is deactivated and the second is operated in the normal mode and is thus utilized for power generation, correspondingly changed control of the fuel injection into the second cylinder is necessary in order to obtain a torque or power which corresponds to the torque or the power that would be output by the internal combustion engine if the first cylinder were also being operated in the normal mode. It is possible in this way to avoid less efficient part-load operation of the second cylinder.
The internal combustion engine preferably comprises an even number of cylinders, in particular at least four cylinders, wherein a selective cylinder deactivation is realized by opening of the outlet valve of each odd-numbered cylinder during a compression stroke of the respective cylinder. The odd-numbered cylinders are therefore utilized not for power generation but rather for enriching the exhaust-gas flow with fuel, whereas the even-numbered cylinders are operated in the normal mode and generate the mechanical power for driving the drivetrain and the odd-numbered cylinders. A particularly efficient enrichment of the exhaust-gas flow with fuel is attained in this way. As described above, the selective cylinder deactivation may be performed for a number of strokes or for a time period which is dependent on the enrichment of the exhaust gas required for the operation of the exhaust-gas aftertreatment device. The selective cylinder deactivation may also be dependent on the power demand or the load of the internal combustion engine.
A system is disclosed herein for a four-stroke internal combustion engine, the system comprising: at least two cylinders; a fuel direct injection device; a variable valve timing system; an engine controller to control spark ignition and valve timing according to load; wherein, below a lower load threshold, a first cylinder is deactivated, an injection of fuel takes place into a combustion chamber of the first cylinder and an outlet valve of the first cylinder is open during a compression stroke. Fuel injected into the deactivated cylinders may enter the exhaust tract through the open outlet valves and serve as a reducing agent therein.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.